More specifically, the invention relates to a magnet arrangement in a magnetic resonance apparatus with a permanent magnet system for generating a homogeneous magnetic field in the direction of a z axis in a measuring volume, the permanent magnet system having magnet elements, pole piece elements and yoke elements of magnetic material arranged cylinder-symmetrically with respect to the z axis.
A magnet arrangement of this kind is known from U.S. Pat. No. 7,084,633 B2.
Magnetic resonance (including nuclear magnetic resonance (NMR) and including electron spin resonance (ESR)) is a widely used measuring method with which chemical compounds can be analysed. In the area of NMR spectroscopy and in MR imaging, a very homogeneous and temporally constant magnetic field is required in a sample volume to be defined, said magnetic field being able to be generated with resistive or supraconductive coils or with a suitable permanent magnet arrangement. The use of permanent magnets is preferred if flux densities of well below 2T are sufficient and if a relatively compact structure is desired.
To maximize the magnetic flux in the sample volume and thus at the same time to minimize the stray flux, the magnetic flux has to be focussed. In this context, a distinction is made between magnetic circuits in which the magnetic return path is through a yoke made of a soft-magnetic material, or those magnetic circuits that do not need a yoke. The latter are in most cases modifications of so-called Halbach magnets, where the return is simplified by a gradual change of the direction of magnetization. In practice, a Halbach array is typically obtained through a stepwise change of the direction of magnetization. This departure from the ideal Halbach magnetization contributes to increasing the stray field outside the actual magnet arrangement and makes additional shielding necessary for applications that are free of stray field. In yoke-based magnets, sufficient shielding of the stray field can be achieved well below the saturation flux density through skilful design of the yoke and use of the yoke material.
The permanent magnets are typically held by a support structure, which can additionally perform the function of the magnetic return path. Traditional designs have a yoke in the form of a rectangular frame (window frame, H-yoke, C-yoke), with two central magnet assemblies arranged opposite each other, between which the sample volume is located. On account of their open structure, these yoke designs are also often affected by stray fields that extend far beyond the outer contours of the magnet arrangement.
To achieve the field homogeneity that is required in the sample volume for NMR measurements, the yoke-free Halbach magnets have to be provided with correction mechanisms in order to be able to compensate for tolerances of the magnetic material or the position of the individual magnet blocks, and this complicates the mechanical construction. Yoke-based magnets generally have a parallel pair of pole pieces made of a soft-magnetic material with suitably high saturation flux density. By suitable selection of the geometry of the pole pieces and special treatment of the surfaces, the field profile can be optimized in a comparatively simple and efficient way.
U.S. Pat. No. 8,077,002 B2 discloses a permanent magnet device for MRI applications. It involves a pair of solid disc-shaped magnets which in parallel delimit the air gap as plane of symmetry and each comprise an offset (protruding) ring magnet, such that an L-shaped magnet is obtained. Pole pieces are arranged on both sides of the air gap for the parallel orientation of the magnetic field in the air gap. The so-called L-magnet arrangement is held by a T-yoke consisting of the components 114a,b and 110a,b. Since the magnet arrangement in question is an open structure, this T-yoke is necessary for structural reasons. The described T-yoke lines are connected by an additional web as magnetic return path.
U.S. Pat. No. 7,084,633 B2, cited in the introduction, discloses a magnet arrangement for MRT appliances using permanent magnets. The permanent magnets are arranged in such a way that a pair of central permanent magnets are connected to respective pole pieces, the measuring volume being located in the space between them. The magnetization of the two magnets is oriented in the same direction, such that the magnetic flux flows through the measuring volume in a defined direction. Further segments of permanent magnets are arranged in a circular shape around said magnets, wherein the direction of magnetization of the magnets is oriented radially outwards and radially inwards, such that the magnetic field is strengthened within the measuring volume. To return the magnetic field, plates made of ferromagnetic material are arranged to form a yoke. For structural reasons, and in order to guide the magnetic flux of the corner segments, supporting yokes are required at the respective corners.
However, all of the known permanent magnet systems for magnetic resonance have the following disadvantages, some of them serious:                Generation of a considerable magnetic stray field in the external environment of the MR apparatus        System-related large volume of the known permanent magnet systems and, therefore, large amount of space needed and high expenditure in terms of material        Easy displaceability of the system components relative to each other on account of the high magnetic forces that occur, and resulting problems as regards long-term operation, and expensive design measures for safely preventing displacement.        
As regards the last-mentioned point, it should be noted that the manufacturers of the known designs in some cases (have to) take considerable extra effort to reliably ensure that nothing is displaced. This requires design measures, for example reinforcements, supports, etc., which generally make the structure considerably heavier and larger. Displacement of individual assemblies may adversely affect the homogeneity of the magnet or, in the worst case scenario, damage objects within the sample volume (risk of crushing). Not least, movable assemblies may also cause a problem during fitting, e.g. because housing elements are then still incomplete.